This invention relates to synthetic cordierite-containing ceramic (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) bodies having low values of thermal expansion, and also relates to a method for producing the bodies.
The attractiveness of ceramics as materials for components of high temperature engines is widely recognized. For example, the heat exchanger is an integral part of gas turbine engine designs. The purpose of the heat exchanger is to recover waste heat losses and to preheat the incoming air in order to improve efficiency of engine operation. The level of both fuel consumption, and noxious exhaust emissions are expected to be significantly lower than for conventional internal combustion engines. Ceramic heat exchangers have the advantage of high temperature capabilities which has a direct effect on engine efficiency. Ceramics are also lighter in weight than metals and have the potential for lower cost.
Increased efforts in the last few years on gas turbine engine development have initiated research on ceramic materials possessing the necessary thermal shock and corrosion resistance, as well as adequate mechanical strength, density and thermal stability. Considerable attention has been focused on lithia-alumina-silica (LAS) and magnesia-alumina-silica (MAS) ceramics for these applications because of their low thermal expansion coefficients and correspondingly high thermal shock resistance. At present, MAS materials appear to exhibit better chemical durability than LAS materials in the particularly corrosive environment of high temperature combustion engine operation.
Early work on MAS was directed to producing bodies having a high percentage of synthetic cordierite and to characterizing the phase relationships and crystal structures of the resulting materials. An extensive review of early work is contained in Synthetic Cordierite, M. E. Tyrrell et al., Bulletin 594 of the Bureau of Mines, U.S. Government Printing Office, Washington, D.C., 1961. Tyrrell et al. were primarily concerned with achieving efficient synthesis of dense MAS bodies containing high percentages of cordierite and high dielectric constants. During the course of their investigation, techniques for producing slags consistently high in cordierite were developed, and are described in U.S. Pat. No. 2,731,355. The best thermal expansion coefficients reported are about 1.84 centimeters per centimeter per degree Cx10.sup.6 between 20.degree.C and 800.degree.C, which is equivalent to about 1424 parts per million within this temperature range.
Similar studies to obtain dense synthetic cordierite bodies having high dielectric constants are reported in Inorganic Dielectric Research, E. J. Smoke, Research and Development Technical Report ECOM -- 0232 -- 5, U.S. Army Electronics Command, Fort Monmouth, N.J., December, 1969. Thermal expansion data indicated values were obtained which were equivalent to at least about 1160 parts per million within the range 25.degree.C to 800.degree.C which is expected for conventional cordierite bodies. The calculated value for polycrystalline cordierite bodies from lattice thermal expansion data obtained by G. R. Fisher et al. (American Crystallographic Association Meeting, University Park, Pennsylvania 1974) using X-ray diffraction techniques is 1120 parts per million within the range 25.degree.C to 800.degree.C.
In view of the recent emphasis upon obtaining ceramic materials possessing excellent thermal shock resistance for use in high temperature engine applications, recent efforts have been directed toward lowering the thermal expansion coefficients of MAS bodies in order to meet the thermal shock requirements imposed by the intended applications.